1. Field of the Invention
This invention relates to a system for controlling an internal combustion engine for motor vehicles equipped with an automatic transmission and more particularly relates to such a system for controlling an automotive engine output power in such a manner as to reduce an unpleasant shock experienced by the vehicle occupant at gear shifting.
2. Description of the Prior Art
In motor vehicles equipped with an automatic transmission, several speed gears each having a multi-disc hydraulic clutch are selectively engaged in sequence in accordance with vehicle operating conditions including for instance a vehicle road speed and an opening degree of the throttle valve. More specifically, solenoid valves are provided in the hydraulic passage such that the electrohydraulic valves will drain pressurized oil from one clutch and apply the next one in sequence when a shift controller determines gear shifting in response to the aforesaid vehicle speed and the throttle valve opening. In order to ensure smooth shifting, the hydraulic pressure must be suitably controlled for the clutches concerned and if failed, the engine tends to race undesirably. Further, due to a sudden change in clutch slippage, it may happen to cause the vehicle occupant to feel an unpleasant shock, so-called "jolting". Namely, when said one clutch being drained its oil as aforementioned, the clutch will begin to disengage and to slip. Assume that a case is the downshifting. If the next lower gear brought into engagement late, the engine speed rises excessively in the meanwhile. The racing engine is then coupled to the next gear, which causes a shock similar to that happens in a situation where a vehicle is struck from behind. Alternatively, if the gear is engaged too fast, the engine speed, still remaining in a relatively low engine speed, will be suddenly pulled up to the speed of the lower gear, which generates another shock like braking. Those shocks are called as the jolting as aforementioned. Although such a shock may happen when the vehicle is equipped with a manually shifted transmission, it will be less serious in the kind of the vehicle, since the driver is able to be skilled enough for avoiding occurrence of the shock.
In view of the above, attempts have been made to control engine output power during shifting so as to reduce the shock. An example of one such system was given in U.S. Pat. No. 4,266,447. In the system disclosed, an engine control unit, upon receipt of a shift signal from the shift controller, kept to reduce the engine output power for a period which was determined on the engine speed at that instance. The conventional system, however, had a drawback that it was so complicated in system structure. That is, the shift signal had to be supplied from the shift controller to the unit, which required troublesome signal processing in the unit. Another and more serious problems encountered in the system was that, the power transfer in the transmission was not monitored when determining engine output control. And instead, the state was presumed outside the transmission through the engine speed, which disadvantageously led it difficult to cope with a sudden change in the power transmission. Furthermore, since the control was initiated in the conventional system only when the shift signal was transmitted from the shift controller, the system was unable to avoid a shock caused by clutch slippage due to a reason other than the gear shifting and was accordingly helpless in preventing the vehicle occupant from feeling discomfort and also ineffective in avoiding an undue wear to clutch plates resulting from the clutch slippage. The prior art system, thus, leaves much to be desired.